1. Field of the Invention
This invention pertains to evaporating a solid target material by means of a laser that is resonant with a vibrational mode in the material and depositing the material on a substrate.
2. Description of Related Art
Infrared pulsed laser deposition (PLD) was first reported in 1960's but did not emerge as a thin film coating technology at that time for number of reasons. These include the slow repetition rate of the available lasers, and the lack of commercially available high power lasers. At that time, infrared PLD used infrared laser light of 1.06 μm which was not resonant with any single photon absorption band of the material being deposited. Although PLD developed through the years it was not until late 1980's that ultraviolet PLD became popular with the discovery of complex superconducting ceramics and the commercial availability of high energy, high repetition rate lasers. Ultravioler PLD is now a common laboratory technique used for the production of a broad range of thin film materials.
Ultraviolet PLD has been an extremely successful technique for the deposition of thin films of a large variety of complex, multi-component inorganic materials. Ultraviolet PLD has also been applied to the growth of thin polymeric and organic films, with varying degrees of success. It has been shown that polymethyl methacrylate, polytetrafluoroethylene and polyalphamethyl styrene undergo rapid depolymerization during ultraviolet laser ablation, with the monomer of each strongly present in the ablation plume. The photochemical modification occurs because the energy of the ultraviolet laser causes the irradiated material to be electronically excited. The geometry of the excited electronic state can be very different from the ground electronic state. Relaxation of the excited state can be to either the ground state of the starting material, or a the ground state of a geometrically different material. Deposited films are therefore photochemically modified from the starting material, showing a dramatic reduction in the number average molecular weight. For these polymers, depositing the film at an elevated substrate temperature can increase the molecular weight distribution of the deposited thin film material. On arrival, monomeric material repolymerizes on the heated substrate surface, with degree of repolymerization being determined by the thermal activity of the surface. Therefore, even in some of the most successful cases of ultraviolet PLD, the intense interaction between the target material and laser leads to chemical modification of the polymer.
An alternative approach to PLD of polymeric materials with ultraviolet lasers is matrix-assisted pulsed laser evaporation, disclosed in U.S. Pat. No. 6,025,036 and others, in which roughly 0.1–1% of a polymer material to be deposited is dissolved in an appropriate solvent an frozen to form an ablation target. The ultraviolet laser light interacts mostly with the solvent and the guest material is evaporated much more gently than in conventional PLD. While this technique can produce smooth and uniform polymer films, it requires that the polymer of interest be soluble in a non-interacting solvent. Finding a suitable solvent system that is also non-photochemically active is a significant challenge and limits the usefulness of the technique. There are examples where electronic excitation of the solvent/polymer system has been observed to produce undesirable photochemical modification of the polymer, such as reduction in the average weight average molecular weight. An additional disadvantage of the matrix-assisted pulsed laser evaporation is that the deposition rate is about an order of magnitude lower than conventional PLD, which can render matrix-assisted pulsed laser evaporation ineffective for applications that require thick, i.e., greater than about 1 micron, coatings.
The ability to deposit polymeric materials in the form of a thin film is important for a wide range of uses including electronics, chemical sensors, photonics, analytical chemistry and biological sciences and technologies. The term “thin film” is a term of art typically denoting films as thin as a few molecules thick and up to a few microns in thickness, generally from about 10 angstroms to about 1 micron.
An important biomedical application of polymer thin films is for biocompatible polymer thin films on drug particles. The coating serves to both delay and regulate the release of the drug in the body. Two techniques that have been demonstrated in the coating of drug particles which include wet chemical technique and a vapor deposition technique. In the wet chemical technique, the coated particle can be more than 50% coating on weight bases. A coating that minimizes the coating to drug weight ratio is desired for obvious reasons. It is also important to control the thickness of the deposited film since control of the dissolution rate governs the rate of drug delivery. While UVPLD has been used to deposit much thinner (on the order of a few hundred angstroms) coatings on drug particles, the deposition process introduces significant and undesirable chemical modification in the coating material as a consequence of the ultraviolet excitation.